"(ESA) made plans to return to the Jupiter system and Io. In 2009, NASA approved a plan to send an orbiter to Europa called the ""Jupiter Europa Orbiter"" as part of a joint program with ESA called the Europa/Jupiter System Mission. The ESA component of the project was the ""Jupiter Ganymede Orbiter"". However, the EJSM mission collaboration was cancelled. ESA is continuing with its initiative under the name Jupiter Icy Moon Explorer (JUICE) to explore Ganymede, Europa, and Callisto, without plans to investigate Io at all. The proposed NASA Discovery mission ""Io Volcano Observer"", currently going through a competitive process"
"moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei on 8 January 1610, and possibly independently by Simon Marius. The first reported observation of Io and Europa was made by Galileo Galilei on 7 January 1610 using a 20×-magnification refracting telescope at the University of Padua. However, in that observation, Galileo could not separate Io and Europa due to the low magnification of his telescope, so that the two were recorded as a single point of light. The following day, 8 January 1610 (used as the discovery date for Europa by the IAU), Io and Europa were seen for"
"other two have lights on them which are gradually spreading. These three bodies are the moons Io, Callisto, and Ganymede, the latter two of which are presently being colonised by humans. Humans have been attempting to explore Europa ever since Lucifer was created in 2010. However, none of these attempts has been successful. Every probe that has attempted to land on Europa has been destroyed on approach. The debris from every probe falls to the surface of the planet, and the debris from some of the first ships to be destroyed is venerated by the Europans. Finally, there is a"
"leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa. Various space probes from ""Pioneers 10"" and ""11"" to ""Galileo"" and ""Cassini"" have studied Callisto. Because of its low radiation levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system. Callisto was discovered by Galileo in January 1610, along with the three other large Jovian moons—Ganymede, Io, and Europa. Callisto is named after one of Zeus's many lovers in Greek mythology. Callisto was a nymph (or, according to"
"younger, but still ancient, regions with a large array of grooves and ridges. Ganymede has a high number of craters, but many are gone or barely visible due to its icy crust forming over them. The satellite has a thin oxygen atmosphere that includes O, O, and possibly O (ozone), and some atomic hydrogen. Callisto (Jupiter IV) is the fourth and last Galilean moon, and is the second largest of the four, and at 4820.6 kilometers in diameter, it is the third largest moon in the Solar System, and barely smaller than Mercury, though only a third of the latter's"
"be reflected in a flattening and decline of brightness, while solar forcing should be reflected in a flattening and then resumed rise of brightness. Ten of the many natural satellites in the Solar System are known to have atmospheres: Europa, Io, Callisto, Enceladus, Ganymede, Titan, Rhea, Dione, Triton and Earth's Moon. Ganymede and Europa both have very tenuous oxygen atmospheres, thought to be produced by radiation splitting the water ice present on the surface of these moons into hydrogen and oxygen. Io has an extremely thin atmosphere consisting mainly of sulfur dioxide (), arising from volcanism and sunlight-driven sublimation of"
"the expansion continued, but some of the angular moment was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered the 2:1 resonance with Ganymede. Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance. Ganymede is the largest and most massive moon in the Solar System. Its diameter of 5,268 km is 0.41 times that of Earth, 0.77 times that of Mars, 1.02 times that of Saturn's Titan (the second-largest moon), 1.08 times Mercury's, 1.09 times Callisto's, 1.45 times Io's and 1.51"
"it was found that Europa has primarily amorphous ice, Ganymede has both amorphous and crystalline ice, and Callisto is primarily crystalline. This is thought to be the result of competing forces: the thermal crystallization of amorphous ice versus the conversion of crystalline to amorphous ice by the flux of charged particles from Jupiter. Closer to Jupiter than the other three moons, Europa receives the highest level of radiation and thus through irradiation has the most amorphous ice. Callisto is the furthest from Jupiter, receiving the lowest radiation flux and therefore maintaining its crystalline ice. Ganymede, which lies between the two,"
"ions farther from the planet, where they are implanted preferentially on the trailing hemispheres of Europa and Ganymede. On Callisto however, for unknown reasons, sulfur is concentrated on the leading hemisphere. Plasma may also be responsible for darkening the moons' trailing hemispheres (again, except Callisto's). Energetic electrons and ions, with the flux of the latter being more isotropic, bombard surface ice, sputtering atoms and molecules off and causing radiolysis of water and other chemical compounds. The energetic particles break water into oxygen and hydrogen, maintaining the thin oxygen atmospheres of the icy moons (since the hydrogen escapes more rapidly). The"
"to eclipse. Different versions of Eclipse have been given different science-related names. The versions named after Callisto, Europa, and Ganymede, which are moons of Jupiter, were followed by a version named after Galileo the discoverer of those moons. These were followed by two sun-themed names, Helios of Greek mythology, and Indigo, one of the seven colors of a rainbow (which is produced by the sun). The version after that, Juno, has a triple meaning: a Roman mythological figure, an asteroid, and a spacecraft to Jupiter. Kepler, Luna, and Mars continued the astronomy theme, and then Neon and Oxygen constitutes a"
"that their mean density decreases with distance from Jupiter. Callisto, the outermost and least dense of the four, has a density intermediate between ice and rock whereas Io, the innermost and densest moon, has a density intermediate between rock and iron. Callisto has an ancient, heavily cratered and unaltered ice surface and the way it rotates indicates that its density is equally distributed, suggesting that it has no rocky or metallic core but consists of a homogeneous mix of rock and ice. This may well have been the original structure of all the moons. The rotation of the three inner"
"moons Io, Europa and Ganymede. The orbital semimajor axis is only 0.13 AU, around a third of the average distance between Mercury and the Sun, and is more eccentric than the orbit of any of the major planets of our solar system. Despite this, it is located in the inner regions of the system's habitable zone, since Gliese 876 is such an intrinsically faint star. A limitation of the radial velocity method used to detect Gliese 876 c is that only a lower limit on the planet's mass can be obtained. This is because the measured mass value depends on"
"objectives for Ganymede, and to a lesser extent for Callisto, are: For Europa, the focus is on the chemistry essential to life, including organic molecules, and on understanding the formation of surface features and the composition of the non-water-ice material. Furthermore, JUICE will provide the first subsurface sounding of the moon, including the first determination of the minimal thickness of the icy crust over the most recently active regions. More distant spatially resolved observations will also be carried for several minor irregular satellites and volcanic moon Io. The main spacecraft design drivers are related to the large distance to the"
"Europa (moon) </math></ref> Europa ( ) (Jupiter II) is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet. It is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the mythological mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman god Jupiter). Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and has a water-ice crust and probably an iron–nickel core. It has a very thin atmosphere composed primarily of oxygen. Its"
"late October 2015 to mid-January 2016. During the first month of flight, the probe's main structures would be deployed, the nuclear reactor activated, and the thrusters tested. Interplanetary flight would have lasted until April 2021 (the ion engines were supposed to work two thirds of the time). Once the probe was in the influence area of Jupiter, the navigation would become more complex and difficult. The probe would have to use gravity assist maneuvers to enter orbit. The probe would have studied Callisto and then Ganymede for three months each, and finally Europa for one month (studies of Io were"
"fourth on 13 January. Galileo named the group of four the ""Medicean stars"", in honour of his future patron, Cosimo II de' Medici, Grand Duke of Tuscany, and Cosimo's three brothers. Later astronomers, however, renamed them ""Galilean satellites"" in honour of their discoverer. These satellites were independently discovered by Simon Marius on 8 January 1610 and are now called Io, Europa, Ganymede, and Callisto the names given by Marius in his ""Mundus Iovialis"" published in 1614. His observations of the satellites of Jupiter caused a revolution in astronomy: a planet with smaller planets orbiting it did not conform to the"
"degrees to the left or right of these moons in its orbit. Callisto, still further out, would appear only a quarter the size of Io. None of the surface features on Jupiter's moons would appear as prominent as the lunar maria do when the Moon is viewed from Earth. Dark and light patches would be visible on Io's surface due to the coloration of sulfur that covers Io, and the largest volcanoes would be designated by dark points, but the lack of large, contrasting features results in a poor view of it. Europa, however, would appear as a completely featureless"
"In February–March 2007, the ""New Horizons"" probe on its way to Pluto obtained new images and spectra of Callisto. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. Several close flybys of Callisto are planned during the mission. Formerly proposed for a launch in 2020, the Europa Jupiter System Mission (EJSM) was a joint NASA/ESA proposal for exploration of Jupiter's moons. In February 2009 it was announced that ESA/NASA had given this mission priority ahead of the Titan Saturn System Mission. ESA's contribution still faced funding"
"will be the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022, followed by NASA's Europa Clipper mission in 2025. There has been great interest in studying the icy moons in detail because of the possibility of subsurface liquid oceans on Jupiter's moons Europa, Ganymede, and Callisto. Funding difficulties have delayed progress. NASA's ""JIMO"" (""Jupiter Icy Moons Orbiter"") was cancelled in 2005. A subsequent proposal was developed for a joint NASA/ESA mission called EJSM/Laplace, with a provisional launch date around 2020. EJSM/Laplace would have consisted of the NASA-led Jupiter Europa Orbiter and the ESA-led Jupiter Ganymede"
"only being heated by radioactive elements, whereas Europa has tidal energy as well, from its greater proximity to Jupiter. Based on the considerations mentioned above and on other scientific observations, it is thought that of all of Jupiter's moons, Europa has the greatest chance of supporting microbial life. The ""Pioneer 10"" and ""Pioneer 11"" Jupiter encounters in the early 1970s contributed little new information about Callisto in comparison with what was already known from Earth-based observations. The real breakthrough happened later with the ""Voyager 1"" and ""Voyager 2"" flybys in 1979. They imaged more than half of the Callistoan surface"
"and possibly a JAXA-led Jupiter Magnetospheric Orbiter. ESA's contribution faced funding competition from other ESA projects, but on 2 May 2012 the European part of the mission, renamed Jupiter Icy Moon Explorer (JUICE), obtained a L1 launch slot in 2022 with an Ariane 5 in the ESA's Cosmic Vision science programme. The spacecraft will orbit Ganymede and conduct multiple flyby investigations of Callisto and Europa. The Russian Space Research Institute is currently evaluating the Ganymede Lander (GL) mission, with emphasis on astrobiology. The Ganymede Lander would be a partner mission for JUpiter ICy moon Explorer (JUICE). If selected, it would"
"white disc. Even when viewed from nearby, most spacecraft images use contrast enhancements to clearly show the cracks in the Europan ice, which are only about 10–15 percent darker than the rest of the surface. Vague dark and light patches would be visible on Ganymede, whereas Callisto is much too distant for any features to be made out. All four Galilean moons stand out because of the swiftness of their motion, compared to the Moon. They are all also large enough to fully eclipse the Sun. Because Jupiter's axial tilt is minimal, and the Galilean moons all orbit in the"
"thin atmosphere made up mostly of sulfur dioxide (SO). If a surface data or collection vessel were to land on Io in the future, it would have to be extremely tough (similar to the tank-like bodies of the Soviet Venera landers) to survive the radiation and magnetic fields that originate from Jupiter. Europa (Jupiter II), the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter, which is slightly smaller than the Moon. The name comes from a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the"
"slightly less the accepted modern value. These measurements allowed astronomers to estimate Io's density, given as 2.88 g/cm following the Beta Scorpii occultation. While this is 20% less than the currently accepted value, it was enough for astronomers to note the differences between the densities of the inner two Galilean satellites (Io and Europa) versus the outer two Galilean satellites (Ganymede and Callisto). The densities of Io and Europa suggested that they were composed primarily of rock while Ganymede and Callisto contained more ices. Beginning in the 1890s, larger telescopes allowed astronomers to directly observe large scale features on the"
"is 77% the diameter of Mars. Ganymede's size made it a popular location for early science fiction authors looking for locations beyond Mars that might be inhabitable by humans. In reality, Ganymede is a cold, icy, cratered world with a vanishingly thin atmosphere. Callisto is the outermost of the Galilean satellites. It is a large moon, only slightly smaller than the planet Mercury. It is cold, icy, and heavily cratered, with a very tenuous atmosphere. Despite its size and early discovery, it has not been featured in fiction as much as the other Galilean satellites. Amalthea, or Jupiter V, is"
"Gaspra, and discovered the first asteroid moon, Dactyl, around 243 Ida. In 1994, ""Galileo"" observed Comet Shoemaker–Levy 9's collision with Jupiter. Jupiter's atmospheric composition and ammonia clouds were recorded, the clouds possibly created by outflows from the lower depths of the atmosphere. Io's volcanism and plasma interactions with Jupiter's atmosphere were also recorded. The data ""Galileo"" collected supported the theory of a liquid ocean under the icy surface of Europa, and there were indications of similar liquid-saltwater layers under the surfaces of Ganymede and Callisto. Ganymede was shown to possess a magnetic field and the spacecraft found new evidence for"
"for comparison with other more active and complex worlds. It is speculated that there could be life in Callisto's subsurface ocean. Like Europa and Ganymede, as well as Saturn's moons Enceladus, Mimas, Dione and Titan, a possible subsurface ocean might be composed of salt water. It is possible that halophiles could thrive in the ocean. As with Europa and Ganymede, the idea has been raised that habitable conditions and even extraterrestrial microbial life may exist in the salty ocean under the Callistoan surface. However, the environmental conditions necessary for life appear to be less favorable on Callisto than on Europa."
"on the icy crust of Europa. The presence of the minerals may have been the result of a collision with an asteroid or comet. Some scientists have speculated that life on Earth could have been blasted into space by asteroid collisions and arrived on the moons of Jupiter in a process called lithopanspermia. Europa (moon) </math></ref> Europa ( ) (Jupiter II) is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet. It is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa,"
"times the Moon's. Its mass is 10% greater than Titan's, 38% greater than Callisto's, 66% greater than Io's and 2.02 times that of the Moon. The average density of Ganymede, 1.936 g/cm, suggests a composition of about equal parts rocky material and mostly water-ices. The mass fraction of ices is between 46–50 %, which is slightly lower than that in Callisto. Some additional volatile ices such as ammonia may also be present. The exact composition of Ganymede's rock is not known, but is probably close to the composition of L/LL type ordinary chondrites, which are characterized by less total iron,"
"moons - Ganymede, Europa and Io. Numerical integration indicates that the coplanar, four-planet system is stable for at least another billion years. This planetary system comes close to a triple conjunction between the three outer planets once per orbit of the outermost planet. The outermost three of the known planets likely formed further away from the star, and migrated inward. Gliese 876d, discovered in 2005, is the innermost known planet. With an estimated minimum mass only 5.88 times that of the Earth, it is possible that it is a dense terrestrial planet. Gliese 876 c, discovered in 2001, is a"
"a 4:1 mean-motion orbital resonance with Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede. This resonance helps maintain Io's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity. Without this forced eccentricity, Io's orbit would circularize through tidal dissipation, leading to a geologically less active world. Like the other Galilean satellites and the Moon, Io rotates synchronously with its orbital period, keeping one face nearly pointed toward Jupiter. This synchrony provides the definition for Io's longitude system. Io's prime meridian"
"has an ionosphere associated with its atmosphere is unresolved. Ganymede's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610. The satellite's name was soon suggested by astronomer Simon Marius, after the mythological Ganymede, cupbearer of the Greek gods, kidnapped by Zeus for the purpose. Beginning with ""Pioneer 10"", several spacecraft have explored Ganymede. The Voyager probes, ""Voyager 1"" and ""Voyager 2"", refined measurements of its size, while ""Galileo"" discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer"
"the lowest value among the gas giants in part because it has the lowest bulk density (). Ganymede has the lowest moment of inertia factor among solid bodies in the Solar System because of its fully differentiated interior, a result in part of tidal heating due to the Laplace resonance, as well as its substantial component of low density water ice. Callisto is similar in size and bulk composition to Ganymede, but is not part of the orbital resonance and is less differentiated. The Moon is thought to have a small core, but its interior is otherwise relatively homogenous. The"